The present invention relates to novel processes for the preparation of high purity aluminas, and more particularly, it relates to novel processes for the preparation of high purity aluminas from impure solutions of aluminum chloride.
The high purity aluminas which are aimed at have purities above 99.9% Al.sub.2 O.sub.3. Their uses today are quite numerous. For example, such high purity aluminas are used in the area of ceramic processes, the preparation of translucent aluminas, in luminescent compositions for fluorescent lights, in bioceramics, and for metal polishing. In addition, they comprise a primary material for the preparation of single crystals according to the Verneuil crystallization technique.
Different processes can be used to obtain aluminas of high purity. Certain among these utilize aluminum as the starting material. In such cases, this is converted to the salt of an organic acid or to an alcoholate which is then hydrolyzed or thermally decomposed to obtain finally the alumina. Despite the possibility of recycling the alcohol in certain cases, the cost of this alumina is very high because of the necessity to employ metallic aluminum.
Some of the other routes start from impure aluminum hydroxide products, large quantities of which are obtained in the aluminum industry starting from minerals. These consist of passing through an intermediate mineral salt the crystallization of which permits the elimination of the major portion of the impurities. The product thus obtained also is thereafter thermally decomposed to form pure alumina. Some other procedures according to this principle are based on the crystallization of ammonium alum which is formed starting from aluminum hydroxide, sulfuric acid, and ammonia, but this method of operation with such salt presents numerous disadvantages.
In the first place, it leads to a very high consumption of reactants since the molecule NH.sub.4 Al(SO.sub.4).sub.2.12H.sub.2 O does not contain more than 11% alumina and the gaseous decomposition products are not recoverable. Moreover, ammonium alum presents the peculiarity of melting at about 90.degree. in its own water of crystallization. This results in certain technological difficulties in economically carrying out the thermal treatment of this product, although a distinctly better performing process is the subject of French Patent Application No. 80/14620.
By comparison, it seems more attractive to go by the aluminum chloride route. In effect, by starting from aluminum chloride solutions, it is possible to crystallize the hexahydrate, AlCl.sub.3.6H.sub.2 O, in which the amount of alumina is raised to about 21% and which does not melt in its own water of crystallization. Moreover, the pyrohydrolysis of this salt liberates a mixture of hydrochloric acid and water which can eventually be recovered.
The sources of aluminum chloride are, moreover, rather numerous. Certain of these solutions are obtained by attacking aluminum with hydrochloric acid such as is practiced in the processes called etching. These liquids are of good purity, although occasionally relatively dilute. The dissolution of certain minerals, such as clays, equally permits the preparation of impure chloride solutions, the purification of which has been studied. Finally, as for alum, it is easy to dissolve the trihydrate of alumina in hydrochloric acid solutions to obtain an aluminum chloride solution. The ways of changing solutions of aluminum chloride to alumina are numerous.
Certain of the methods for converting aluminum chloride to alumina involve a direct pyrohydrolysis of the solution in a flame. The reaction is extremely fast but the dechlorohydration is rarely complete. Also, the alumina recovered always contains relatively large quantities of residual chlorine. To reduce the chlorine content, one can only increase the flame temperature which, concomitantly, enlarges the crystals and this inevitably leads to powders having a small specific surface area. Moreover, the brevity of treatment, although helpful to equipment productivity, is bad for the homogeneity of the product.
The other category of processses includes those which call for the crystallized aluminum chloride hexahydrate to pass through an intermediate stage. This step in principle permits the separation of impurities and especially iron, but the chloride remains dissolved in the mother liquor. After washing with a pure acid solution, the crystals are thermally treated. There follows a hydrolysis by the water of crystallization which results in partial dechlorohydration of the product. The complete pyrohydrolysis requires finally bringing the product to a sufficiently high temperature.
It is still observed that it has not been possible up to the present to prepare transition aluminas with a high specific surface and not containing more than about 0.1% residual chlorine on an industrial scale by such methods. Now, for certain uses of high purity aluminas, it is necessary to provide products having specific areas greater than 100 m.sup.2 /g. This is the case for polishing agents, for the Verneuil crystallization, for use as loading, especially in electrical capacitor papers, and for other uses.
Moreover, for ceramic procedures involving corundum fritting, one always seeks to obtain powders of the highest possible homogeneity in particle size. To obtain such uniformity, it is preferable to prepare a homogeneous product at an intermediate stage, crystallized in fine elementary particles by which the transformation to corundum can be assured by a well defined calcination schedule.
There accordingly exists a need to have a commercial process for the production of high purity aluminas starting from aluminum chloride solutions offering the means for obtaining intermediate aluminas characterized by a high specific surface area and a residual chlorine content of not more than 0.1%.